Voltage source and current source with capacitor

ABSTRACT

In a voltage source according to the present invention, a switching transistor (Q 1 ) is turned on when the voltage of an electrolytic capacitor (C 1 ) exceeds a predefined positive voltage level (El), to keep the electrolytic capacitor (C 1 ) at the predefined voltage level (E 1 ), and turned off when the voltage of the electrolytic capacitor (C 1 ) is reduced below the predefined voltage level (E 1 ). A switching transistor (Q 2 ) is turned on when the voltage of an electrolytic capacitor (C 2 ) is reduced below a predefined negative voltage level (−E 2 ), to keep the electrolytic capacitor (C 2 ) at the predefined voltage level (−E 2 ), and turned off when the voltage of the electrolytic capacitor (C 2 ) exceeds the predefined voltage level (−E 2 ). Therefore, an output voltage (V 3 ) is controlled between the predefined voltage level (E 1 ) as an upper limit and the predefined voltage level (−E 2 ) as a lower limit.

TECHNICAL FIELD

The present invention relates to a voltage source capable of supplying apredetermined level of AC voltage and a current source capable ofsupplying a predetermined level of AC electric current.

BACKGROUND ART

(A) Voltage sources are power supply apparatuses for supplying apredetermined level of AC voltage. Conventional voltage sources includethose of an electromagnetic coil type and a capacitor division type.

The voltage sources of the electromagnetic coil type are disadvantageousfor use as general-purpose equipment because of their heaviness andbulkiness.

The voltage sources of the capacitor division type have a drawback thata desired voltage ratio cannot be provided unless a capacitor having ahigh capacitance as compared with the capacitance of a load is employed.In use with an inductive load, the voltage sources may cause resonanceunless a capacitor having a significantly high capacitance is employed.

It is therefore an object of the present invention to provide alight-weight and compact voltage source. It is another object of theinvention to provide a voltage source in which a voltage can easily beset at any desired level.

(B) Conventional convenient-to-use current sources are of a type whichincludes an iron core and a coil inserted between a power source and aload. The current sources employing the iron core and the coil have asimple circuit configuration, but are disadvantageous because of theirheaviness, bulkiness and greater heat loss.

A high frequency switching circuit may be employed for constructing acurrent source. However, the high frequency switching circuit deals witha frequency much higher than the commercial power source frequency, sothat high frequency noises are radiated to adversely affect peripheralapparatuses.

Therefore, there is a demand for developing a current source capable ofsupplying an AC electric current by employing the commercial powersource frequency on an “as is” basis.

If the current source is designed so that the electric current canvariably be set at any desired level, the usefulness of the currentsource will be enhanced.

It is therefore further another object of the present invention toprovide a light-weight and compact current source which has a simpleconstruction including a switching element and a capacitor, and featuresreduced heat dissipation and no high frequency noise.

It is still another object of the present invention to provide a currentsource in which an electric current can easily be set at any desiredlevel.

DISCLOSURE OF THE INVENTION

(a) A voltage source according to one inventive aspect comprises, asshown in FIG. 1, a serial capacitor C connected in series between an ACpower source V and a load, and a first voltage limiting circuit F1 and asecond voltage limiting circuit F2 connected in parallel to the load.The first voltage limiting circuit F1 includes a first switching elementS1 and a first capacitor C1 connected in series. The second voltagelimiting circuit F2 includes a second switching element S2 and a secondcapacitor C2 connected in series. The first switching element S1 isconstantly conductive to an electric current I1 flowing in one directionfor charging the first capacitor C1 in a positive half cycle of the ACpower source, and is controllably turned on and off for an electriccurrent −I1 flowing in the other direction. When the voltage of thefirst capacitor C1 exceeds a predefined positive voltage level E1, thefirst switching element S1 is turned on to keep the first capacitor C1at the predefined voltage level E1. When the voltage of the firstcapacitor C1 is reduced below the predefined voltage level E1, the firstswitching element S1 is turned off. The second switching element S2 isconstantly conductive to an electric current I2 flowing in one directionfor charging the second capacitor C2 in a negative half cycle of the ACpower source, and is controllably turned on and off for an electriccurrent −I2 flowing in the other direction. When the voltage of thesecond capacitor C2 is reduced below a predefined negative voltage level−E2, the second switching element S2 is turned on to keep the secondcapacitor C2 at the predefined voltage level −E2. When the voltage ofthe second capacitor C2 exceeds the predefined voltage level −E2, thesecond switching element S2 is turned off.

With the aforesaid arrangement, the first capacitor C1 is chargedthrough the switching element S1 in the positive half cycle of the ACpower source. When the voltage of the first capacitor C1 exceeds thepredefined positive voltage level E1, the first switching element S1 isturned on for the reverse electric current −I1 to keep the voltage ofthe first capacitor C1 at the level E1. As a result, an output voltageis limited to a range below the voltage level E1 defined as an upperlimit by the first voltage limiting circuit F1 in the positive halfcycle.

In the negative half cycle of the AC power source, the second capacitorC2 is charged through the second switching element S2. When the voltageof the second capacitor C2 is reduced below the predefined negativevoltage level −E2, the second switching element S2 is turned on for thereverse electric current −I2 to keep the voltage of the second capacitorC2 at the level −E2. As a result, the output voltage is limited to arange above the voltage level −E2 defined as a lower limit by the secondvoltage limiting circuit F2 in the negative half cycle.

Consequently, the output voltage is limited to a range between thevoltage level E1 defined as the upper limit by the first voltagelimiting circuit F1 and the voltage level −E2 defined as the lower limitby the second voltage limiting circuit F2 in a full wave cycle.

The voltage levels E1 and −E2 may be set in a continuously variablemanner. Thus, a variable voltage source can be provided which allows forcontinuous voltage adjustment.

Where electrolytic capacitors which feature a higher capacitance/volumeratio are employed as the first capacitor C1 and the second capacitorC2, a compact voltage source can be provided by taking advantage of thisfeature. The electrolytic capacitors have polarities, and generally havea disadvantage that they cannot be used with alternating current.However, this disadvantage can be eliminated by permitting the firstvoltage limiting circuit F1 to operate in the positive half cycle of theAC power source and permitting the second voltage limiting circuit F2 tooperate in the negative half cycle of the AC power source.

According to this inventive aspect, the output voltage can be setbetween the positive voltage level defined as the upper limit by thefirst voltage limiting circuit and the negative voltage level defined asthe lower limit by the second voltage limiting circuit in the full wavecycle. These voltage limiting circuits can each easily be constructed byemploying the voltage source, the switching element and the electrolyticcapacitor, so that a small-size and light-weight voltage source can beprovided.

(b) A current source according to another inventive aspect comprises, asshown in FIG. 4, a switching element S1 and a capacitor C1 connected inseries between an AC power source V and a load. The switching element S1is constantly conductive to an electric current I1 flowing in onedirection for charging the capacitor C1 in a positive half cycle of theAC power source V, and is controllably turned on and off for an electriccurrent −I1 flowing in the other direction. The switching element S1 iscontrolled to be turned on when a voltage between opposite terminals ofthe capacitor C1 is higher than a predefined voltage level E1, and to beturned off when the voltage of the capacitor C1 is not higher than thepredefined voltage level E1.

It is assumed that the voltage of the load is negligibly small ascompared with the voltage of the power source. With the aforesaidarrangement, the capacitor C1 is charged through the switching elementS1 in the positive half cycle of the AC power source voltage V. When theAC voltage V starts decreasing from a peak value, the capacitor C1starts discharging. However, when the voltage of the capacitor C1 isreduced to not higher than the predefined positive voltage level E1, theswitching element S1 is turned off for the electric current −I1 flowingin the other direction to keep the voltage of the capacitor C1 at thelevel E1.

In the subsequent cycles, the capacitor C1 is charged and dischargedonly when the AC power source voltage V exceeds the predefined voltagelevel E1, and the voltage of the capacitor C1 is kept at the level E1when the AC power source voltage V is reduced to not higher than thepredefined voltage level E1.

A load electric current is determined by differentiating the voltage ofthe capacitor C1. The electric current occurs only when the capacitor C1is charged and discharged. Since the charging and discharging of thecapacitor C1 occur only when the AC power source voltage V exceeds thepredefined voltage level E1, the electric current occurs only at thistime. The level of the electric current varies as a function of thepredefined voltage level E1. As the predefined voltage level E1 isreduced, the charging and discharging period of the capacitor C1 isincreased and, hence, the electric current level is increased. As thepredefined voltage level E1 is increased, the charging and dischargingperiod of the capacitor C1 is reduced and, hence, the electric currentlevel is reduced. Thus, the electric current level can be controlled bycontrolling the predefined voltage level E1.

Where the predefined voltage level E1 is continuously variable, avariable current source can be provided which allows for continuoussetting.

Where an electrolytic capacitor which features a highercapacitance/volume ratio is employed as the capacitor C1, the currentsource has a compact structure by taking advantage of this feature. Theelectrolytic capacitor has polarities, and generally has a disadvantagethat it cannot be used with alternating current. However, thisdisadvantage can be eliminated by preventing the voltage of thecapacitor C1 from decreasing below the predefined voltage level E1.

(c) A current source according to further another inventive aspectcomprises, as shown in FIG. 5, a switching element S2 and a capacitor C2connected in series between an AC power source V and a load. Theswitching element S2 is constantly conductive to an electric current I2flowing in one direction for charging the capacitor C2 in a negativehalf cycle of the AC power source V, and is controllably turned on andoff for an electric current −I2 flowing in the other direction. Theswitching element S2 is controlled to be turned on when a voltagebetween opposite terminals of the capacitor C2 is lower than apredefined negative voltage level −E2, and to be turned off when thevoltage of the capacitor C2 is not lower than the predefined voltagelevel −E2.

It is assumed that the voltage of the load is negligibly small ascompared with the voltage of the power source. With the aforesaidarrangement, the capacitor C2 is charged through the switching elementS2 in the negative half cycle of the AC power source voltage V. When theAC voltage V starts increasing from a negative peak value, the capacitorC2 starts discharging. However, when the voltage of the capacitor C2 isincreased to not lower than the predefined negative voltage level −E2,the switching element S2 is turned off for the electric current −I2flowing in the other direction to keep the voltage of the capacitor C2at the level −E2.

In the subsequent cycles, the capacitor C2 is charged and dischargedonly when the AC power source voltage V is reduced below the predefinedvoltage level −E2. When the AC power source voltage V is increased tonot lower than the predefined voltage level −E2, the voltage of thecapacitor C2 is kept at the level −E2.

On the other hand, a load electric current is determined bydifferentiating the voltage of the capacitor C2. The electric currentoccurs only when the AC power source voltage V is lower than thepredefined voltage level −E2. The level of the electric current variesas a function of the predefined voltage level −E2. As the absolute valueof the predefined voltage level −E2 is reduced, the charging anddischarging period of the capacitor C2 is increased and, hence, theelectric current level is increased. As the absolute value of thepredefined voltage level −E2 is increased, the charging and dischargingperiod of the capacitor C2 is reduced and, hence, the electric currentlevel is reduced. Thus, the electric current level can be controlled bycontrolling the predefined voltage level −E2.

Where the predefined voltage level −E2 is continuously variable, avariable current source can be provided which allows for continuoussetting.

Where an electrolytic capacitor which features a highercapacitance/volume ratio is employed as the capacitor C2, the currentsource has a compact structure by taking advantage of this feature. Theelectrolytic capacitor has polarities, and generally has a disadvantagethat it cannot be used with alternating current. However, thisdisadvantage can be eliminated by preventing the voltage of thecapacitor C2 from increasing above the predefined voltage level −E2.

(d) A current source according to still another inventive aspectcomprises, as shown in FIG. 6, a switching element S1 and a capacitor C1connected in series between an AC power source V and a load, and aswitching element S2 and a capacitor C2 connected in series between theAC power source V and the load.

The switching element S1 is constantly conductive to an electric currentI1 flowing in one direction for charging the capacitor C1 in a positivehalf cycle of the AC power source V, and is controllably turned on andoff for an electric current −I1 flowing in the other direction. Theswitching element S1 is controlled to be turned on when a voltagebetween opposite terminals of the capacitor C1 is higher than apredefined voltage level E1, and to be turned off when the voltage ofthe capacitor C1 is not higher than the predefined voltage level E1.

The switching element S2 is constantly conductive to an electric currentI2 flowing in one direction for charging the capacitor C2 in a negativehalf cycle of the AC power source V, and is controllably turned on andoff for an electric current −I2 flowing in the other direction. Theswitching element S2 is controlled to be turned on when a voltagebetween opposite terminals of the capacitor C2 is lower than apredefined negative voltage level −E2, and to be turned off when thevoltage of the capacitor C2 is not lower than the predefined voltagelevel −E2.

This arrangement provides a full-wave type current source which employsthe current source (b) and the current source (c) in combination. As aresult, an output electric current is controlled on the basis of thepredefined voltage level E1 in the positive half cycle, and controlledon the basis of the predefined voltage level −E2 in the negative halfcycle.

The voltage levels E1 and −E2 may be set in a continuously variablemanner. In this case, a variable current source can be provided whichallows for continuous adjustment of the electric current.

Where electrolytic capacitors which feature a higher capacitance/volumeratio are employed as the capacitors C1 and C2, the current source has acompact structure by taking advantage of this feature. The electrolyticcapacitors have polarities, and generally have a disadvantage that theycannot be used with alternating current. However, this disadvantage canbe eliminated by keeping the voltage of the capacitor C1 at not lowerthan the predefined voltage level E1 and keeping the voltage of thecapacitor C2 at not higher than the predefined voltage level −E2.

(e) A current source according to further another inventive aspectcomprises, as shown in FIG. 7, a switching element S1 and a capacitor C1connected in series between an AC power source V and a load, and a diodeD1 and a reference capacitor C01 connected in series between the ACpower source V and the load. The diode D1 has such an orientation thatthe reference capacitor C01 is charged in a positive half cycle of theAC power source V. The switching element S1 is constantly conductive toan electric current I1 flowing in one direction for charging thecapacitor C1 in the positive half cycle of the AC power source V, and iscontrollably turned on and off for an electric current −I1 flowing inthe other direction. The switching element S1 is controlled to be turnedon when the voltage V1 of the capacitor C1 is higher than the sum of apeak value Vp of the power source voltage and a predefined negativevoltage level −E1, and to be turned off when the voltage V1 of thecapacitor C1 is not higher than the level Vp−E1.

With the aforesaid arrangement, the capacitor C1 is charged through theswitching element S1 and the reference capacitor C01 is charged throughthe diode D1 in the positive half cycle of the AC power source voltageV. When the AC voltage V starts decreasing from the peak value, thecapacitor C1 starts discharging. However, the reference capacitor C01has no discharge circuit and, hence, does not discharge. Therefore, thevoltage V01 of the reference capacitor C01 is kept at the peak value Vpof the AC voltage V. When the voltage V1 of the capacitor C1 is reducedto not higher than the sum Vp−E1 of the peak value Vp and the predefinedvoltage level −E1, the switching element S1 is turned off to keep thevoltage V1 of the capacitor C1 at the level Vp−E1.

In the subsequent cycles, the capacitor C1 is charged and dischargedwhen the AC power source voltage V is higher than the sum of the peakvalue Vp and the predefined voltage level −E1, i.e., when the followingexpression (1) is satisfied:V>Vp−E 1   (1)

The voltage of the capacitor C1 is kept at the level Vp−E1 when the ACpower source voltage V is not higher than the sum of the peak value Vpand the predefined voltage level −E1, i.e., when the followingexpression (2) is satisfied:V≦Vp−E 1   (2)

A load electric current is determined by differentiating the voltage ofthe capacitor C1. The electric current occurs only when the capacitor C1is charged and discharged. Since the charging and discharging of thecapacitor C1 occur when the AC power source voltage V satisfies theabove expression (1), the electric current occurs at this time. Thelevel of the electric current varies as a function of the predefinedvoltage level −E1. As the absolute value of the predefined voltage level−E1 is increased, the charging and discharging period of the capacitorC1 is increased and, hence, the electric current level is increased. Asthe absolute value of the predefined voltage level −E1 is reduced, thecharging and discharging period of the capacitor C1 is reduced and,hence, the electric current level is reduced. Thus, the electric currentlevel can be controlled by controlling the predefined voltage level −E1.

According to this inventive aspect, the reference capacitor C01 isadditionally provided, and a voltage difference between the voltage V1of the capacitor C1 and the voltage V01 of the reference capacitor C01is utilized for the control. Therefore, the electric current level canstably be controlled even if the voltage VL of the load fluctuates.

Where the predefined voltage level −E1 is continuously variable, avariable current source can be provided which allows for continuoussetting.

Where an electrolytic capacitor which features a highercapacitance/volume ratio is employed as the capacitor C1, the currentsource has a compact structure by taking advantage of this feature.

(f) A current source according to still another inventive aspectcomprises, as shown in FIG. 8, a switching element S2 and a capacitor C2connected in series between an AC power source V and a load, and a diodeD2 and a reference capacitor C02 connected in series between the ACpower source V and the load. The diode D2 has such an orientation thatthe reference capacitor C02 is charged in a negative half cycle of theAC power source V.

The switching element S2 is constantly conductive to an electric currentI2 flowing in one direction for charging the capacitor C2 in thenegative half cycle of the AC power source V, and is controllably turnedon and off for an electric current −I2 flowing in the other direction.The switching element S2 is controlled to be turned on when the voltageV2 of the capacitor C2 is lower than the sum −Vp+E2 of a negative peakvalue −Vp of the power source voltage and a predefined voltage level E2,and to be turned off when the voltage V2 of the capacitor C2 is notlower than the level −Vp+E2.

A load voltage is represented by VL. With the aforesaid arrangement, thecapacitor C2 is charged through the switching element S2 and thereference capacitor C02 is charged through the diode D2 in the negativehalf cycle of the AC power source voltage V. When the AC voltage Vstarts increasing from the peak value, the capacitor C2 startsdischarging. However, the reference capacitor C02 has no dischargecircuit. Therefore, the voltage V02 of the reference capacitor C02 iskept generally constant at the voltage level −Vp. When the voltage V2 ofthe capacitor C2 is increased to not lower than the level −Vp+E2, theswitching element S2 is turned off to keep a differential voltage V2−V02is kept at a level −E2. Therefore, the voltage V2 of the capacitor C2 isthereafter kept generally constant.

In the subsequent cycles, the capacitor C2 is charged when the powersource voltage V is lower than the level −Vp+E2, i.e., when thefollowing expression (3) is satisfied:V<−Vp+E 2   (3)

The voltage of the capacitor C2 is kept at the level −Vp+E2 when thepower source voltage V is not lower than the level −Vp+E2, i.e., whenthe following expression (4) is satisfied:V≧Vp+E 1   (4)

A load electric current is determined by differentiating the voltage ofthe capacitor C2. The electric current occurs only when the capacitor C2is charged and discharged. Since the charging and discharging of thecapacitor C2 occur when the AC power source voltage V satisfies theabove expression (3), the electric current occurs at this time. Thelevel of the electric current varies as a function of the predefinedvoltage level E2. As the absolute value of the predefined voltage levelE2 is reduced, the charging and discharging period of the capacitor C2is reduced and, hence, the electric current level is reduced. As theabsolute value of the predefined voltage level E2 is increased, thecharging and discharging period of the capacitor C2 is increased and,hence, the electric current level is increased. Thus, the electriccurrent level can be controlled by controlling the predefined voltagelevel E2.

According to this inventive aspect, the reference capacitor C02 isadditionally provided, and the voltage difference between the voltage V2of the capacitor C2 and the voltage V02 of the reference capacitor C02is utilized for the control. Therefore, the electric current level canstably be controlled even if the load voltage VL fluctuates.

Where the predefined voltage level E2 is continuously variable, avariable current source can be provided which allows for continuoussetting.

Where an electrolytic capacitor which features a highercapacitance/volume ratio is employed as the capacitor C2, the currentsource has a compact structure by taking advantage of this feature.

(g) A current source according to further another inventive aspectcomprises, as shown in FIG. 9, a switching element S1 and a capacitor C1connected in series between an AC power source V and a load, and a diodeD1 and a reference capacitor C01 connected in series between the ACpower source V and the load. The diode D1 has such an orientation thatthe reference capacitor C01 is charged in a positive half cycle of theAC power source V. The current source further comprises a switchingelement S2 and a capacitor C2 connected in series between the AC powersource V and the load, and a diode D2 and a reference capacitor C02connected in series between the AC power source V and the load. Thediode D2 has such an orientation that the reference capacitor C02 ischarged in a negative half cycle of the AC power source V.

The switching element S1 is constantly conductive to an electric currentI1 flowing in one direction for charging the capacitor C1 in thepositive half cycle of the AC power source V, and is controllably turnedon and off for an electric current −I1 flowing in the other direction.The switching element S1 is controlled to be turned on when the voltageof the capacitor C1 is higher than the sum Vp−E1 of a peak value Vp ofthe power source voltage and a predefined voltage level −E1, and to beturned off when the voltage of the capacitor C1 is not higher than thelevel Vp−E1. The switching element S2 is constantly conductive to anelectric current I2 flowing in one direction for charging the capacitorC2 in the negative half cycle of the AC power source V, and iscontrollably turned on and off for an electric current −I2 flowing inthe other direction. The switching element S2 is controlled to be turnedon when the voltage V2 of the capacitor C2 is lower than the sum −Vp+E2of a negative peak value −Vp of the power source voltage and apredefined voltage level E2, and to be turned off when the voltage V2 ofthe capacitor C2 is not lower than the level −Vp+E2.

This arrangement provides a full-wave type current source which employsthe current source (e) and the current source (f) in combination. As aresult, an output electric current is controlled on the basis of thepredefined voltage level −E1 in the positive half cycle, and controlledon the basis of the predefined voltage level E2 in the negative halfcycle.

The voltage levels −E1 and E2 may be set in a continuously variablemanner. In this case, a variable current source can be provided whichallows for continuous adjustment of the electric current.

Where electrolytic capacitors which feature a higher capacitance/volumeratio are employed as the capacitors C1 and C2, the current source has acompact structure by taking advantage of this feature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit configuration diagram for explaining the principleof a voltage source according to the present invention;

FIG. 2 is a more specific circuit diagram of the voltage sourceaccording to the invention;

FIGS. 3(a) to 3(d) are waveform diagrams of the voltage V of a powersource, the voltage V1 of an electrolytic capacitor C1, the voltage V2of an electrolytic capacitor C2 and an output voltage V3;

FIG. 4 is a circuit configuration diagram for explaining the principleof a current source (half wave type) according to the invention;

FIG. 5 is a circuit configuration diagram for explaining the principleof another current source (half wave type) according to the invention;

FIG. 6 is a circuit configuration diagram for explaining the principleof further another current source (full wave type) according to theinvention;

FIG. 7 is a circuit configuration diagram for explaining the principleof still another current source (half wave type) employing a referencecapacitor according to the invention;

FIG. 8 is a circuit configuration diagram for explaining the principleof further another current source (half wave type) employing a referencecapacitor according to the invention;

FIG. 9 is a circuit configuration diagram for explaining the principleof still another current source (full wave type) employing a referencecapacitor according to the invention;

FIG. 10 is a circuit configuration diagram of a current source (fullwave type) according to an embodiment of the present invention;

FIGS. 11(a) to 11(c) are waveform diagrams of the voltage V of a powersource, the voltage V1 of an electrolytic capacitor C1, the voltage V2of an electrolytic capacitor C2 and an output current I;

FIG. 12 is a circuit configuration diagram of a current source (fullwave type) employing a reference capacitor according to anotherembodiment of the invention; and

FIGS. 13(a) to 13(c) are waveform diagrams of the voltage V of a powersource, the voltage V1 of an electrolytic capacitor C1, the voltage V2of an electrolytic capacitor C2 and an output current I.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will hereinafter be described indetail with reference to the attached drawings.

(1) FIRST EMBODIMENT

FIG. 2 is a circuit diagram of a variable voltage source (full wavetype) according to the present invention.

A capacitor C is connected in series to an AC power source. A firstvoltage limiting circuit F1 and a second voltage limiting circuit F2 areconnected in parallel on an output side of the capacitor C, andconnected to output terminals. A load ZL is connected to the outputterminals.

The first voltage limiting circuit F1 is adapted to limit a voltage in apositive half cycle, and includes a switching transistor Q1 and anelectrolytic capacitor C1 connected in series. The switching transistorQ1 has polarities such that its collector is connected to the capacitorC and its emitter is connected to the electrolytic capacitor C1. A diodeD1 is connected in parallel to the switching transistor Q1 between theemitter and collector of the switching transistor Q1. The other terminalof the electrolytic capacitor C1 is grounded. A constant voltage diodeZD1 is connected between a base of the switching transistor Q1 and aresistor division point a. At this resistor division point a, resistorsR11 and R12 connected in parallel to the electrolytic capacitor C1 areconnected to each other. The resistor R12 is continuously variable.

The second voltage limiting circuit F2 is adapted to limit a voltage ina negative half cycle, and includes a switching transistor Q2 and anelectrolytic capacitor C2 connected in series. The switching transistorQ2 has polarities such that its collector is connected to the capacitorC and its emitter is connected to the electrolytic capacitor C2. A diodeD2 is connected in parallel to the switching transistor Q2 between theemitter and collector of the switching transistor Q2. The other terminalof the electrolytic capacitor C2 is grounded. A constant voltage diodeZD2 is connected between a base of the switching transistor Q2 and aresistor division point b. At this resistor division point b, resistorsR21 and R22 connected in parallel to the electrolytic capacitor C2 areconnected to each other. The resistor R22 is continuously variable.

The switching transistors Q1 and Q2 are a PNP transistor and an NPNtransistor, respectively. The electrolytic capacitor C1 is connected tothe switching transistor Q1 at its positive terminal, while theelectrolytic capacitor C2 is connected to the switching transistor Q2 atits negative terminal. The diode D1 is connected in such an orientationthat the electrolytic capacitor C1 is charged in the positive halfcycle, while the diode D2 is connected in such an orientation that theelectrolytic capacitor C2 is charged in the negative half cycle. Theconstant voltage diode ZD1 is connected to the base of the switchingtransistor Q1 at its positive terminal, while the constant voltage diodeZD2 is connected to the base of the switching transistor Q2 at itsnegative terminal. The voltage of the positive terminal of the constantvoltage diode ZD1 is represented by E1, and the voltage of the negativeterminal of the constant voltage diode ZD2 is represented by −E2. Thevoltage levels E1 and −E2 can be set in a continuously variable mannerby the resistors R12 and R22, respectively.

In the aforesaid circuit, the capacitor C, the electrolytic capacitor C1and the electrolytic capacitor C2 have device constants of, for example,C=20 μ, C1=100 μF and C2=100 μF, respectively.

Next, an explanation will be given to the operation of the aforesaidvariable voltage source.

FIGS. 3(a) to 3(d) are waveform diagrams of a power source voltage V, avoltage V1 between the opposite terminals of the electrolytic capacitorC1, a voltage V2 between the opposite terminals of the electrolyticcapacitor C2 and an output voltage V3 between the output terminals(referred to simply as “output voltage”). A reference character t on theabscissa denotes time. The positive half cycles of the power sourcevoltage V are represented by P1, P2 and the like, and the negative halfcycles of the power source voltage V are represented by N1, N2 and thelike.

It is assumed that the power source is turned on at a time point t=0.The charging of the electrolytic capacitor C1 through the diode D1 isstarted in the first positive half cycle P1. The charging time constantof the electric capacitor C1 is set relatively great, so that thevoltage V1 of the electrolytic capacitor C1 does not reach the voltagelevel E1 in the positive half cycle P1. The output voltage V3 has thesame waveform as the voltage V1 of the electrolytic capacitor C1 (seeFIGS. 3(b) and 3(d)).

In the negative half cycle N1, the electrolytic capacitor C2 is chargedthrough the diode D2. In this case, also, the voltage V2 of theelectrolytic capacitor C2 does not reach the voltage level −E2 in thenegative half cycle N1. The output voltage V3 has the same waveform asthe voltage V2 of the electrolytic capacitor C2 (see FIGS. 3(c) and3(d)).

In each of the subsequent several positive half cycles, the charging ofthe electrolytic capacitor C1 is resumed. In one of these half cycles,the voltage V1 of the electrolytic capacitor C1 reaches the voltagelevel E1. At this time point, electrical continuity is establishedbetween the base and emitter of the switching transistor Q1, so that theswitching transistor Q1 is turned on. Thus, the voltage V1 of theelectrolytic capacitor C1 is kept at the predefined voltage level E1.The predefined voltage level E1 is the upper limit of the collectorvoltage of the switching transistor Q1, i.e., the output voltage V3.

In each of the subsequent negative half cycles, the charging of theelectrolytic capacitor C2 is resumed. In the midst of these half cycles,the voltage V2 of the electrolytic capacitor C2 reaches the voltagelevel −E2. At this time point, electrical continuity is establishedbetween the base and emitter of the switching transistor Q2, so that theswitching transistor Q2 is turned on. Thus, the voltage V2 of theelectrolytic capacitor C2 is kept at the predefined voltage level −E2.The predefined voltage level −E2 is the lower limit of the collectorvoltage of the switching transistor Q2, i.e., the output voltage V3.

In the constant state cycles, the output voltage V3 alternatelyoscillates between the predefined voltage level E1 as the upper limitand the predefined voltage level −E2 as the lower limit.

In this way, the upper and lower limits of the power source voltage Vcan be defined. The upper limit voltage level and the lower limitvoltage level are determined by adjusting the resistors R12 and R22 orby setting the breakdown voltages of the constant voltage diodes ZD1 andZD2.

While the voltage source of the present invention has thus beendescribed by way of the embodiment thereof, the invention is not limitedto this embodiment. For example, a half-wave type voltage source can beprovided by employing either the first voltage limiting circuit F1 orthe second voltage limiting circuit F2 shown in FIG. 2.

(2) SECOND EMBODIMENT

FIG. 10 is a circuit diagram of a variable current source (full wavetype) according to the present invention.

Two variable current circuits (a positive half-wave variable currentcircuit G1 and a negative half-wave variable current circuit G2) areconnected in parallel between an AC power source V and a load.

The positive half-wave variable current circuit G1 is adapted to limitan electric current in a positive half cycle, and includes a switchingtransistor Q1 and an electrolytic capacitor C1 connected in series. Theswitching transistor Q1 has polarities such that its collector isconnected to the AC power source V and its emitter is connected to theelectrolytic capacitor C1. A diode D3 is connected in parallel to theswitching transistor Q1 between the collector and emitter of theswitching transistor Q1. One terminal of a voltage source B1 isconnected to a terminal of the electrolytic capacitor C1 on a load side,and a voltage comparator A1 is connected between the other terminal ofthe voltage source B1 and the electrolytic capacitor C1. A comparisonoutput of the voltage comparator A1 is connected to a base of theswitching transistor Q1.

The negative half-wave variable current circuit G2 is adapted to limitan electric current in a negative half cycle, and includes a switchingtransistor Q2 and an electrolytic capacitor C2 connected in series. Theswitching transistor Q2 has polarities such that its collector isconnected to the AC power source V and its emitter is connected to theelectrolytic capacitor C2. A diode D4 is connected in parallel to theswitching transistor Q2 between the collector and emitter of theswitching transistor Q2. One terminal of a voltage source B2 isconnected to a terminal of the electrolytic capacitor C2 on a load side,and a voltage comparator A2 is connected between the other terminal ofthe voltage source B2 and the other terminal of the electrolyticcapacitor C2. A comparison output of the voltage comparator A2 isconnected to a base of the switching transistor Q2.

The switching transistors Q1 and Q2 are a PNP transistor and an NPNtransistor, respectively. The electrolytic capacitor C1 is connected tothe load at its negative terminal, and to the switching transistor Q1 atits positive terminal. The electrolytic capacitor C2 is connected to theload at its positive terminal, and to the switching transistor Q2 at itsnegative terminal. The diode D3 is connected in such an orientation thatthe electrolytic capacitor C1 is charged in the positive half cycle,while the diode D4 is connected in such an orientation that theelectrolytic capacitor C2 is charged in the negative half cycle. Thevoltage source B1 is connected to the voltage comparator A1 at itspositive terminal, and the voltage source B2 is connected to the voltagecomparator A2 at its negative terminal. The voltage comparator A1applies a positive output to the base of the switching transistor Q1when the voltage V1 of the electrolytic capacitor C1 is reduced belowthe voltage E1 of the voltage source B1. The voltage comparator A2applies a negative output to the base of the switching transistor Q2when the voltage V2 of the electrolytic capacitor C2 is increased abovethe voltage −E2 of the voltage source B2.

The voltage level E1 of the voltage source B1 and the voltage level −E2of the voltage source B2 can each be set in a continuously or discretelyvariable manner by known means such as a constant voltage diode and aresistor division circuit not shown.

Next, an explanation will be given to the operation of the aforesaidvariable current source.

FIGS. 11(a) to 11(c) are waveform diagrams of the voltage V of the powersource, the voltage V1 of the electrolytic capacitor C1, the voltage V2of the electrolytic capacitor C2 and an output current I. A referencecharacter t on the abscissa denotes time. The positive half cycles ofthe power source voltage V are represented by P1, P2 and the like, andthe negative half cycles of the power source voltage V are representedby N1, N2 and the like.

It is assumed that the power source is turned on at a time point t=0.The charging of the electrolytic capacitor C1 through the diode D3 isstarted in the first positive half cycle P1. The power source voltage Vreaches a peak value Vp, and then starts decreasing from the peak valueVp. When the power source voltage V reaches the voltage level E1, theswitching transistor Q1 is turned off by the operation of the voltagecomparator A1. The electrolytic capacitor C1 stops discharging, and itsvoltage is kept at the level E1.

In the negative half cycle N1, the electrolytic capacitor C2 is chargedthrough the diode D4. The power source voltage V reaches a peak value−Vp, and then starts increasing from the peak value −Vp. When the powersource voltage V reaches the voltage level −E2, the switching transistorQ2 is turned off by the operation of the voltage comparator A2. Theelectrolytic capacitor C2 stops discharging, and its voltage is kept atthe level −E2.

When the power source voltage V exceeds the voltage level E1 in themidst of the next positive half cycle P2, the charging of theelectrolytic capacitor C1 is resumed.

When the power source voltage V is reduced below the voltage level −E2in the midst of the next negative half cycle N2, the charging of theelectrolytic capacitor C2 is resumed.

In this way, the charging and discharging of the electrolytic capacitorsC1 and C2 are carried out only when the power source voltage V is higherthan the voltage level E1 and is lower than the voltage level −E2. Theelectric current I flows when the electrolytic capacitors C1 and C2 arecharged and discharged as shown in FIG. 11(c). The level of the electriccurrent I is determined by the settings of the voltage levels E1 and−E2. By variably setting the voltage levels E1 and −E2, the electriccurrent level I is varied.

(3) THIRD EMBODIMENT

FIG. 12 is a circuit diagram of a variable current source (full wavetype) according to the present invention.

Two variable current circuits (a positive half-wave variable currentcircuit G1 and a negative half-wave variable current circuit G2) areconnected in parallel between an AC power source V and a load.

The positive half-wave variable current circuit G1 is adapted to limitan electric current in a positive half cycle, and includes a switchingtransistor Q1 and an electrolytic capacitor C1 connected in series. Theswitching transistor Q1 has polarities such that its collector isconnected to the AC power source V and its emitter is connected to theelectrolytic capacitor C1. A diode D3 is connected in parallel to theswitching transistor Q1 between the collector and emitter of theswitching transistor Q1. Further, a terminal of the electrolyticcapacitor C1 on a load side is connected to a reference electrolyticcapacitor C01 and a voltage source B1, and a voltage comparator A1 isconnected between the other terminal of the voltage source B1 and theelectrolytic capacitor C1. A comparison output of the voltage comparatorA1 is connected to a base of the switching transistor Q1.

The negative half-wave variable current circuit is adapted to limit anelectric current in a negative half cycle, and includes a switchingtransistor Q2 and an electrolytic capacitor C2 connected in series. Theswitching transistor Q2 has polarities such that its collector isconnected to the AC power source V and its emitter is connected to theelectrolytic capacitor C2. A diode D4 is connected in parallel to theswitching transistor Q2 between the collector and emitter of theswitching transistor Q2. A terminal of the electrolytic capacitor C2 ona load side is connected to a reference electrolytic capacitor C02 and avoltage source B2, and a voltage comparator A2 is connected between theother terminal of the voltage source B2 and the electrolytic capacitorC2. A comparison output of the voltage comparator A2 is connected to abase of the switching transistor Q2.

The switching transistors Q1 and Q2 are a PNP transistor and an NPNtransistor, respectively. The electrolytic capacitor C1 and theelectrolytic capacitor C01 are connected to the load at their negativeterminals. The electrolytic capacitor C2 and the electrolytic capacitorC02 are connected to the load at their positive terminals. The diode D3is connected in such an orientation that the electrolytic capacitor C1is charged in the positive half cycle, while the diode D4 is connectedin such an orientation that the electrolytic capacitor C2 is charged inthe negative half cycle. The voltage source B1 is connected to thevoltage comparator A1 at its negative terminal, and the voltage sourceB2 is connected to the voltage comparator A2 at its positive terminal.The voltage comparator A1 applies a positive output to the base of theswitching transistor Q1 when the voltage V1 of the electrolyticcapacitor C1 is reduced below a voltage level at a terminal of thevoltage comparator A1 connected to the voltage source B1. The voltagecomparator A2 applies a negative output to the base of the switchingtransistor Q2 when the voltage V2 of the electrolytic capacitor C2 isincreased above a voltage level at a terminal of the voltage comparatorA2 connected to the voltage source B2.

The voltage level −E1 of the voltage source B1 and the voltage level E2of the voltage source B2 can each be set in a continuously or discretelyvariable manner by known means such as a constant voltage diode and aresistor division circuit not shown.

Next, an explanation will be given to the operation of the aforesaidvariable current source.

FIGS. 13(a) to 13(c) are waveform diagrams of the voltage V of the powersource, the voltage V1 of the electrolytic capacitor C1, the voltage V2of the electrolytic capacitor C2 and an output current I. A referencecharacter t on the abscissa denotes time. The positive half cycles ofthe power source voltage V are represented by P1, P2 and the like, andthe negative half cycles of the power source voltage V are representedby N1, N2 and the like.

It is assumed that the power source is turned on at a time point t=0.The charging of the electrolytic capacitor C1 through the diode D3 isstarted in the first positive half cycle P1. At the same time, thereference electrolytic capacitor C01 is charged through the diode D1.The power source voltage V reaches a peak value Vp, and then startsdecreasing from the peak value Vp. At this time, the electrolyticcapacitor C1 starts discharging. However, the reference electrolyticcapacitor C01 does not discharge, but is kept at the peak voltage Vp.When the power source voltage V reaches a voltage level Vp−E1, theswitching transistor Q1 is turned off by the operation of the voltagecomparator A1. Thus, the electrolytic capacitor C1 stops discharging,and its voltage is kept at the level Vp−E1.

In the negative half cycle N1, the electrolytic capacitor C2 is chargedthrough the diode D4. The power source voltage V reaches a peak value−Vp, and then starts increasing from the peak value −Vp. At this time,the electrolytic capacitor C2 starts discharging. However, the referenceelectrolytic capacitor C02 does not discharge, but is kept at the peakvoltage −Vp. When the power source voltage V reaches a voltage level−Vp+E2, the switching transistor Q2 is turned off by the operation ofthe voltage comparator A2. The electrolytic capacitor C2 stopsdischarging, and its voltage is kept at the level −Vp+E2.

When the power source voltage V exceeds the voltage level Vp−E1 in themidst of the next positive half cycle P2, the charging of theelectrolytic capacitor C1 is resumed.

When the power source voltage V is reduced below the voltage level−Vp+E2 in the midst of the next negative half cycle N2, the charging ofthe electrolytic capacitor C2 is resumed.

In this way, the charging and discharging of the electrolytic capacitorsC1 and C2 are carried out only when the power source voltage V is higherthan the voltage level Vp−E1 and is lower than the voltage level −Vp+E2.

The electric current I flows when the electrolytic capacitors C1 and C2are charged and discharged as shown in FIG. 13(c). The level of theelectric current I is determined by the settings of the voltage levels−E1 and E2. By variably setting the voltage levels −E1 and E2, theelectric current level I is varied.

While the embodiments of the present invention have thus been described,the invention is not limited to these embodiments. For example, ahalf-wave type current source according to the present invention can beprovided by employing either the positive half-wave variable currentcircuit G1 or the negative half-wave variable current circuit G2 shownin FIG. 10 or 12.

1. A voltage source comprising a serial capacitor connected in seriesbetween an AC power source and a load, and a first voltage limitingcircuit and a second voltage limiting circuit connected in parallel tothe load, the first voltage limiting circuit comprising a firstswitching element and a first capacitor connected in series, the secondvoltage limiting circuit comprising a second switching element and asecond capacitor connected in series, wherein the first switchingelement is constantly conductive to an electric current flowing in onedirection for charging the first capacitor in a positive half cycle ofthe AC power source, and is controllably turned on and off for anelectric current flowing in the other direction, wherein, when a voltageof the first capacitor exceeds a predefined positive voltage level (E1),the first switching element is turned on to keep the first capacitor atthe predefined positive voltage level (E1) and, when the voltage of thefirst capacitor is reduced below the predefined positive voltage level(E1), the first switching element is turned off, wherein the secondswitching element is constantly conductive to an electric currentflowing in one direction for charging the second capacitor in a negativehalf cycle of the AC power source, and is controllably turned on and offfor an electric current flowing in the other direction, wherein, when avoltage of the second capacitor is reduced below a predefined negativevoltage level (−E2), the second switching element is turned on to keepthe second capacitor at the predefined negative voltage level (−E2) and,when the voltage of the second capacitor exceeds the predefined negativevoltage level (−E2), the second switching element is turned off.
 2. Avoltage source as set forth in claim 1, wherein the predefined positivevoltage level and the predefined negative voltage level are continuouslyvariable.
 3. A voltage source as set forth in claim 1, wherein the firstcapacitor and the second capacitor each comprise an electrolyticcapacitor.
 4. A current source comprising a switching element and acapacitor connected in series between an AC power source and a load,wherein the switching element is constantly conductive to an electriccurrent flowing in one direction for charging the capacitor in a halfcycle of the AC power source, and is controllably turned on and off foran electric current flowing in the other direction, and wherein theswitching element is controlled to be turned on when a voltage of thecapacitor is higher than a predefined voltage level, and to be turnedoff when the voltage of the capacitor is not higher than the predefinedvoltage level.
 5. A current source according to claim 4, wherein theswitching element is constantly conductive to an electric currentflowing in one direction for charging the capacitor in a positive halfcycle of the AC power source, and is controllably turned on and off foran electric current flowing in the other direction, and wherein theswitching element is controlled to be turned on when a voltage of thecapacitor is higher than a predefined voltage level (E1), and to beturned off when the voltage of the capacitor is not higher than thepredefined negative voltage level (E1).
 6. A current source according toclaim 4 wherein the switching element is constantly conductive to anelectric current flowing in one direction for charging the capacitor ina negative half cycle of the AC power source, and is controllably turnedon and off for an electric current flowing in the other direction, andwherein the switching element is controlled to be turned on when avoltage of the second capacitor is lower than a predefined negativevoltage level (−E2), and to be turned off when the voltage of thecapacitor is not lower than the predefined negative voltage level (−E2).7. A current source according to claim 5, wherein the switching elementis a first switching element and the capacitor is a first capacitor,further comprising a second switching element and a f second capacitorconnected in series between an AC power source and a load, wherein thesecond switching element is constantly conductive to an electric currentflowing in one direction for charging the second capacitor in thenegative half cycle of the AC power source, and is controllably turnedon and off for an electric current flowing in the other direction, andwherein the second switching element is controlled to be turned on whena voltage of the second capacitor is lower than a predefined negativevoltage level (−E2), and to be turned off when the voltage of the firstsecond capacitor is not lower than a predefined negative voltage level(−E2).
 8. A current source comprising a switching element and a firstcapacitor connected in series between an AC power source and a load, anda diode and a reference capacitor connected in series between the ACpower source and the load, the diode having such an orientation that thereference capacitor is charged in a positive half cycle of the AC powersource, wherein the switching element is constantly conductive to anelectric current flowing in one direction for charging the firstcapacitor in the positive half cycle of the AC power source, and iscontrollably turned on and off for an electric current flowing in theother direction, and wherein the switching element is controlled to beturned on when a voltage of the first capacitor is higher than a sum(Vp−E1) of a peak value (Vp) of a power source voltage and a predefinednegative voltage level (−E1), and to be turned off when the voltage ofthe first capacitor is not higher than the level (Vp−E1).
 9. A currentsource comprising a first switching element and a first second capacitorconnected in series between an AC power source and a load, and a diodeand a reference capacitor connected in series between the AC powersource and the load, the diode having such an orientation that thereference capacitor is charged in a negative half cycle of the AC powersource, wherein the second switching element is constantly conductive toan electric current flowing in one direction for charging the secondcapacitor in the negative half cycle of the AC power source, and iscontrollably turned on and off for an electric current flowing in theother direction, wherein the switching element is controlled to beturned on when a voltage of the second capacitor is lower than a sum(−Vp+E2) of a negative peak value (−Vp) of the power source voltage anda predefined voltage level (E2), and to be turned off when the voltageof the second capacitor is not lower than the level (−Vp+E2).
 10. Acurrent source of claim 8, wherein the switching element is a firstswitching element, further comprising a second switching element and asecond capacitor connected in series between the AC power source and theload, and a second diode and a second reference capacitor connected inseries between the AC power source and the load, the second diode havingsuch an orientation that the second reference capacitor is charged in anegative half cycle of the AC power source, wherein the second switchingelement is constantly conductive to an electric current flowing in onedirection for charging the second capacitor in the negative half cycleof the AC power source, and is controllably turned on and off for anelectric current flowing in the other direction, wherein the secondswitching element is controlled to be turned on when a voltage of thesecond capacitor is lower than a sum (−Vp+E2) of a negative peak value(−Vp) of the power source voltage and a predefined voltage level (E2),and to be turned off when the voltage of the second capacitor is notlower than the level (−Vp+E2).